Effect of dynamical interactions on integrated properties of globular clusters

TL;DR

This study uses N-body simulations to examine how dynamical interactions, metallicity, and primordial binaries influence the integrated properties of Milky Way globular clusters, revealing that dynamical effects make clusters appear younger and bluer.

Contribution

It introduces detailed N-body models including dynamical interactions and primordial binaries to assess their impact on globular cluster integrated properties, which was not thoroughly explored before.

Findings

Dynamical interactions cause clusters to appear slightly younger and bluer.

Preferential loss of low-mass stars affects integrated colours and spectral features.

Primordial binaries mitigate the effects of dynamical interactions.

Abstract

Globular Clusters (GCs) are generally treated as natural validators of simple stellar population (SSP) models. However, there are still some differences between real GCs and SSPs. In this work we use a direct $N$-body simulation code {\hf Nbody6} to study the influences of dynamical interactions, metallicity and primordial binaries on Milky Way globular clusters' integrated properties. Our models start with $N=100,000$ stars, covering a metallicity range $Z=0.0001-0.02$, a subset of our models contain primordial binaries, resulting in a binary fraction as currently observed at a model age of GCs.Stellar evolution and external tidal field representative for an average Milky Way GC are taken into consideration. The integrated colours and Lick indices are calculated using BaSeL and Bluered stellar spectral libraries separately. By including dynamical interactions, our model clusters show integrated features (colours up to 0.01mag bluer, H$\beta$ up to 0.1\AA $ $ greater and [MgFe]$'$ 0.05\AA $ $ smaller) making the clusters appear slightly younger than the model clusters without dynamical interactions. This effect is caused mainly by the preferential loss of low-mass stars which have a stronger contribution to redder passbands as well as different spectral features compared to higher-mass stars. In addition, this effect is larger at lower metallicities. On the contrary, the incorporation of primordial binaries reduces this effect.